Intrinsic Cutoff Frequency Research Articles

Terahertz Schottky Barrier Diodes Based on Aligned Carbon Nanotube Arrays

Carbon nanotubes (CNTs) with superior electrical characteristics are among the most promising candidate materials for constructing millimeter wave and terahertz wave electronic devices. However, the reported Schottky barrier diodes (SBDs) based on CNTs usually exhibited high series resistance and severe mismatch to lower impedance external circuits, which limits the devices for high frequency applications. In this work, CNT SBDs of lateral structure with a rectify current density of -0.78 mA/μm and a short circuit current responsivity of 6.8 A/W are demonstrated based on aligned carbon nanotube arrays (A-CNTs) with high-density and high-semiconducting purity on a quartz substrate. The A-CNT SBD with a width of 10 μm and an L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ch</sub> of 50 nm under zero bias exhibits an ultra-low intrinsic capacitance of 2.5 fF and an intrinsic cutoff frequency <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> of up to 840 GHz, which suggests that CNT-based SBDs are promising for millimeter wave and terahertz wave applications.

Carbon Nanotube Diodes Operating at Frequencies of Over 50GHz.

Radiofrequency (RF)diodes used for fifth and sixth-generation (5G and 6G) mobile and wireless communication networks generally require ultrahigh cut-off frequencies and high integration densities of devices with different functions on a single chip and at low cost. Carbon nanotube diodes are promising devices for radiofrequency applications, but the cut-off frequencies are still far below the theoretical estimates. Here, a carbon nanotube diode that operates in the millimeter-wave frequency bands and is based on solution-processed, high-purity carbon nanotube network films is reported. The carbon nanotube diodes exhibit an intrinsic cut-off frequency over 100GHz and the as-measured bandwidth can exceed 50GHz at least. Furthermore, The rectification ratio of the carbon nanotube diode by approximately three times by using yttrium oxide for local p-type doping in the diode channel is improved.

Ultralow contact resistance in organic transistors via orbital hybridization

Organic field-effect transistors (OFETs) are of interest in unconventional form of electronics. However, high-performance OFETs are currently contact-limited, which represent a major challenge toward operation in the gigahertz regime. Here, we realize ultralow total contact resistance (Rc) down to 14.0 Ω ∙ cm in C10-DNTT OFETs by using transferred platinum (Pt) as contact. We observe evidence of Pt-catalyzed dehydrogenation of side alkyl chains which effectively reduces the metal-semiconductor van der Waals gap and promotes orbital hybridization. We report the ultrahigh performance OFETs, including hole mobility of 18 cm2 V−1 s−1, saturation current of 28.8 μA/μm, subthreshold swing of 60 mV/dec, and intrinsic cutoff frequency of 0.36 GHz. We further develop resist-free transfer and patterning strategies to fabricate large-area OFET arrays, showing 100% yield and excellent variability in the transistor metrics. As alkyl chains widely exist in conjugated molecules and polymers, our strategy can potentially enhance the performance of a broad range of organic optoelectronic devices.

A Ballistic Transport Nanodevice Based on Graphene Nanoribbon FET by Enhanced Productivity for Both Low-Voltage and Radio-Frequency Scopes

The top performance in both the low-voltage and radio frequency (RF) scopes has been nominated for an unique nanodevice made from a graphene nanoribbon with an extremely short gate length (7.5 nm) in this study. Two distinct material engineering options were used, yielding some interesting outcomes. Due to the use of an ultrascale GNRFET in this study, the band structure non-linearity in the Dirac point and the energy-position dependent effective mass model for dual material gate architectures were examined for the first time. The NEGF formalism is used to carry out both the low-voltage and RF research using a three-dimensional (3D) Poisson equation. Low-voltage high performance has been validated by monitoring the key parameters in the terms of on current (Ion), off current (Ioff), Ion to Ioff ratio, subthreshold swing, and drain induced barrier lowering (DIBL) for the proposed device as compared to other structures under the study. The RF performance is examined by evaluation of essential parameters in the cases of parasitic gate capacitance, intrinsic cut-off frequency, intrinsic delay time, and transconductance. Indeed, a device with a higher source side gate work function than it does on the drain side is proposed will shift the energy band from the device’s half to the drain electrode, altering carrier outflow. In addition, the performance of non-linearity and RF intermodulation distortion has been analyzed for all devices under investigation in order to attain the best attitude toward the suggested device in comparison to the other devices under investigation in this work.

Terahertz metamaterial modulators based on wide-bandgap semiconductor lateral Schottky diodes

With the advent of 6G communications and the constant quest for more bandwidth in wireless technologies, the use of frequency bands lying in the terahertz spectrum becomes inevitable. Efficient high-speed modulation and demodulation techniques are necessary for the development of future terahertz communication systems. However, the speed of state-of-the-art terahertz modulators is limited to MHz-GHz; therefore, far away from the requirements of practical high-speed communication systems. In this work, we discuss that lateral Schottky diodes in wide bandgap semiconductors can enable simultaneous high-speed modulation (intrinsic cut-off frequency &gt; 100 GHz), large modulation depth (&gt;10dB), and low-loss (∼1.5dB) in a metamaterial configuration. These devices are lateral and thus do not require complex semiconductor or electromagnetic design or fabrication. Therefore, the proposed modulator design approach can unlock the potentials of the terahertz band for future 6G wireless communications.

14GHz Schottky Diodes Using a p-Doped Organic Polymer.

The low carrier mobility of organic semiconductors and the high parasitic resistance and capacitance often encountered in conventional organic Schottky diodes hinder their deployment in emerging radio frequency (RF) electronics. Here, these limitations are overcome by combining self-aligned asymmetric nanogap electrodes (≈25nm) produced by adhesion lithography, with a high mobility organic semiconductor, and RF Schottky diodes able to operate in the 5G frequency spectrum are demonstrated. C16 IDT-BT is used, as the high hole mobility polymer, and the impact of p-doping on the diode performance is studied. Pristine C16 IDT-BT-based diodes exhibit maximum intrinsic and extrinsic cutoff frequencies (fC ) of >100and 6GHz, respectively. This extraordinary performance is attributed to the planar nature of the nanogap channel and the diode's small junction capacitance (<2pF). Doping of C16 IDT-BT with the molecular p-dopant C60 F48 improves the diode's performance further by reducing the series resistance resulting to intrinsic and extrinsic fC of >100and ≈14GHz respectively, while the DC output voltage of an RF rectifier circuit increases by a tenfold. Ourwork highlights the importance of the planar nanogap architecture and paves the way for the use of organic Schottky diodes in large-area RF electronics of the future.

High-Frequency Graphene Base Hot-Electron Transistor.

The integration of graphene and other two-dimensional (2D) materials with existing silicon semiconductor technology is highly desirable. This is due to the diverse advantages and potential applications brought about by the consequent miniaturization of the resulting electronic devices. Nevertheless, such devices that can operate at very high frequencies for high-speed applications are eminently preferred. In this work, we demonstrate a vertical graphene base hot-electron transistor that performs in the radio frequency regime. Our device exhibits a relatively high current density (∼200 A/cm2), high common base current gain (α* ∼ 99.2%), and moderate common emitter current gain (β* ∼ 2.7) at room temperature with an intrinsic current gain cutoff frequency of around 65 GHz. Furthermore, cutoff frequency can be tuned from 54 to 65 GHz by varying the collector-base bias. We anticipate that this proposed transistor design, built by the integrated 2D material and silicon semiconductor technology, can be a potential candidate to realize extra fast radio frequency tunneling hot-carrier electronics.

Assessing Negative-Capacitance Drain-Extended Technology for High-Voltage Switching and Analog Applications

In this article, we exploit the negative-capacitance (NC) property of ferroelectrics for high-voltage (HV) applications. We present rigorous analysis and new physical insights into the operation of the NC-based MOSFET with a ferroelectric layer in the gate-stack and drain-extended architecture [NC-drain extended metal oxide semiconductor (DEMOS)]. We show that the voltage amplification effect generated by the NC property of the ferroelectric layer boosts the ON-current and transconductance, which can improve the HV switching and analog performance over conventional DEMOS devices. However, several issues ascribed to the drain-extended structure may arise at large ferroelectric thicknesses, including elevated quasi-saturation and impact ionization effects that limit the benefits of NC. We further investigate the switching and analog figures of merit of NC-DEMOS. We show that the NC-DEMOS offers a smaller ON-resistance without any considerable reduction in the breakdown voltage. Moreover, the NC-DEMOS is found to provide a larger analog gain compared with that of the conventional DEMOS, although with a smaller intrinsic cutoff frequency.

A predictive model for high-frequency operation of two-dimensional transistors from first-principles

First-principles-based device models are in demand in the semiconductor industry to assess the impact of new materials at very early phases of the technology development. Existing models for the 2D metal–oxide–semiconductor field-effect transistor work under quasi-static limit and can only be used for designing circuits operating under half of the transistor's intrinsic cut-off frequency. Here, we develop a compact device model for phosphorene-based transistor that takes into account its band structure anisotropy as well as the carrier inertia, which is crucial for high-frequency operation. In a multi-scale approach, density functional theory based calculation is first carried out to obtain the material specific parameters, which are then used to develop a continuity equation based non-quasi-static model to gain insight into the high-frequency behaviors. We find that channel orientation has a strong impact on both the low and high frequency conductances; however, it affects only the high-frequency component of capacitances. The model is then implemented in an industry-standard circuit simulator using relaxation-time-approximation technique and simulations are conducted to demonstrate its applicability for near cut-off frequency circuit operation. The proposed modeling methodology, which connects material to circuit, thus helps us to expand the design space, where technology downscaling could be very challenging and expensive.

Compact UWB Half-Mode SIW Bandpass Filter With Fully Reconfigurable Single and Dual Notched Bands

Compact ultra-wideband (UWB) bandpass filters based on the half-mode substrate integrated waveguide (SIW) with fully reconfigurable single and dual notched bands are proposed in this article. The lower stopband of the UWB bandpass filter is formed by the intrinsic cutoff frequency of the half-mode SIW, and transmission zeros in the upper stopband are created by utilizing three U-shaped slot resonators. Coupling microstrip resonators loaded by varactor diodes are placed along the open side of the half-mode SIW and microstrip feeding line to construct the in-band notches. By controlling the bias voltages and the size of resonators, fully reconfigurable single and dual notched bands can be obtained. Moreover, the frequency and bandwidth of each notched band can be controlled independently. To demonstrate the proposed technique, five and seven distributed coupling microstrip resonators are first applied to obtain the fully reconfigurable single notched band with the suppression level of 30 and 36 dB, respectively. Then, reconfigurable dual notched bands are demonstrated by changing the bias voltages using the same prototype with seven resonators. At last, nine resonant cells with two sets of different sizes are shown to design reconfigurable dual notched bands with frequency tuning range covering almost the full UWB passband.

100 GHz zinc oxide Schottky diodes processed from solution on a wafer scale

Inexpensive radio-frequency devices that can meet the ultrahigh-frequency needs of fifth- and sixth-generation wireless telecommunication networks are required. However, combining high performance with cost-effective scalable manufacturing has proved challenging. Here, we report the fabrication of solution-processed zinc oxide Schottky diodes that can operate in microwave and millimetre-wave frequency bands. The fully coplanar diodes are prepared using wafer-scale adhesion lithography to pattern two asymmetric metal electrodes separated by a gap of around 15 nm, and are completed with the deposition of a zinc oxide or aluminium-doped ZnO layer from solution. The Schottky diodes exhibit a maximum intrinsic cutoff frequency in excess of 100 GHz, and when integrated with other passive components yield radio-frequency energy-harvesting circuits that are capable of delivering output voltages of 600 mV and 260 mV at 2.45 GHz and 10 GHz, respectively. Nanoscale electrodes fabricated using adhesion lithography can be combined with solution-processed metal oxide semiconductors to create Schottky diodes with performance suitable for 5G communications and beyond.

A Small-Signal GFET Equivalent Circuit Considering an Explicit Contribution of Contact Resistances

A small-signal equivalent circuit for graphene field-effect transistors is proposed considering the explicit contribution of effects at the metal-graphene interfaces by means of contact resistances. A methodology to separate the contact resistances from intrinsic parameters, obtained by a de-embedding process, and extrinsic parameters of the circuit is considered. The experimental high-frequency performance of three devices from two different GFET technologies is properly described by the proposed small-signal circuit. Some model parameters scale with the device footprint. The correct detachment of contact resistances from the internal transistor enables to assess their impact on the intrinsic cutoff frequency of the studied devices.

Effects of Scaling on Analog FoMs of UTBB FD-SOI MOS Transistors: A Detailed Analysis

In this article, the combined effect of BOX thickness (TBOX) and ground-plane (GP) doping (NGP) on channel carrier mobility and analog figures of merit (FoMs) is investigated. It is reported that the thin BOX along with higher NGP will limit the electron/hole mobility of ultrathin body and BOX fully depleted silicon-on-insulator (UTBB FD-SOI) MOS transistors. The physics responsible for this observation is discussed in detail. The contrasting behavior of different GPs, the effect of TBOX scaling, and gate-length scaling on device behavior is also analyzed. It is also shown that in advanced UTBB FD-SOI MOS transistors, a tradeoff exists between transistor intrinsic gain, cutoff frequency, and non-linearity. In nMOS transistors, the best intrinsic gain and cut-off frequency can be achieved with ultrathin BOX and n-type GP (or with no GP), whereas the best linearity can be achieved with ultrathin BOX and p-type GP implant.

Bilayer graphene nanoribbon field-effect transistor with electrically embedded source-side gate

In present-day technology, a bilayer graphene field-effect transistor (BLGNRFET) is known as a suitable alternative for continuing the scaling trend of transistor dimensions. This paper proposes an electrically embedded source-side gate BLGNRFET (EESG-BLGNRFET) in which an appropriate gate engineering takes into account near the source extension. In the proposed structure, this induces a vertical electric field and a larger bandgap in the vicinity of the source for low gate biases reducing off current. The EESG-BLGNRFET benefits from several analog and digital metrics. Intrinsic gain and cut-off frequency 84% and 44% are respectively improved in the EESG-BLGNRFET. Furthermore, the delay and power-delay product (PDP) of the proposed structure are lower than those of the conventional counterpart. It is shown that drain-induced barrier lowing (DIBL) in the EESG-BLGNRFET is decreased that makes the device less sensitive to short-channel effects.

Evaluation of the RF performance of the β-Ga2O3 negative-capacitance field-effect transistor

The performance of the proposed depletion-type β-Ga2O3 negative-capacitance field-effect transistor (NCFET) having stacked ferroelectric HfO2 and Al2O3 gate dielectrics is investigated to determine its utility for radiofrequency (RF) applications. The results obtained for the RF performance of the proposed device are analyzed and compared with those of an experimentally demonstrated β-Ga2O3 metal–oxide–semiconductor field-effect transistor (MOSFET.) The key figures of merit (FOMs) used to investigate its RF performance are the intrinsic capacitances (Cgs and Cgd), transconductance (gm), and cutoff frequency (fT), as well as large-signal FOMs such as the output power (POUT), power gain (GP), and power-added efficiency. The large-signal RF FOMs are obtained by considering a continuous-wave (CW) class A power amplifier, and the measurements are carried out at 0.8 GHz using a passive load and source tuning. The results of this work indicate that the proposed β-Ga2O3 NCFET design will be useful for efficient device integration, offering the advantages of a steep slope and operation at a lower supply voltage for RF applications.

Impact of Different Gate Metals on the RF Performance of Gallium Oxide MOSFET

The performance of gallium oxide MOSFET is limited due to the issues associated with the requirement of good contact materials for gate electrode. The single metal layer at the gate electrode does not possess all the suitable material properties such as good adhesive behavior, high work function, and low specific resistance. In this work, the effect of different metals, to modulate the work function of depletion-type gallium oxide MOSFET is studied so as to improve its RF performance. The advantage of work function modulation and energy band aligned gate electrode shows low specific contact resistivity with a good ohmic contact. The key figure of merits (FOMs) for its RF performance investigation includes intrinsic capacitances (Cgs & Cgd), output conductance (gd), transconductance (gm), and cut-off frequency (fT). This work elucidates that the Ti/Au metal stack with high fT value found to be superior for RF applications.

2 W mm−1 power density of an AlGaN/GaN HEMT grown on free-standing GaN substrate at 40 GHz

In this letter, a record performance at 40 GHz obtained on an AlGaN/GaN high electron mobility transistor (HEMT) grown on hydride vapor phase epitaxy free-standing GaN substrate is reported. An output power density of 2 W mm−1 associated with 20.5% power added efficiency and a linear power gain (Gp) of 4.2 dB is demonstrated for 70 nm gate length device. The device exhibits a maximum DC drain current density of 950 mA mm−1 and a peak extrinsic transconductance (gm Max) of 300 mS mm−1 at VDS = 6 V. A 100 GHz maximum intrinsic cutoff frequency fT, and a maximum intrinsic oscillation frequency fMax of 125 GHz are obtained from S-parameters measurement. This performance is very promising for HEMTs grown on free-standing GaN substrate.

Miniaturized half-mode substrate integrated waveguide diplexer based on SIR–CSRR unit-cell

A miniaturized diplexer based on half-mode substrate integrated waveguide (HMSIW) technology by loading a novel metamaterial unit-cell is proposed. The stepped-impedance resonator (SIR) technique has been used in order to miniaturize the physical size of the conventional complementary split ring resonators (CSRRs). The proposed metamaterial unit-cell, which is called SIR–CSRR, consists of two modified rings which the stepped-impedance slot lines are utilized instead of the conventional slot lines in the CSRRs. The proposed diplexer has been designed by cascading two bandpass filters with different center frequencies. The HMSIW bandpass filters are implemented by etching two SIR–CSRR unit-cells with different sizes. The design procedure is based on the theory of evanescent mode propagation which the SIR–CSRR unit-cells behave as an electric dipoles. A forward-wave passband below the intrinsic cutoff frequency of the HMSIW structure has been achieved by loading the SIR–CSRR unit-cells on the metal surface of the HMSIW structure. This proposed diplexer represent high selectivity and compact size by using of the sub-wavelength resonators. The designed diplexer has been fabricated and experimental verification have been provided. The measured results are in a good agreement with the simulated ones. The total size of the proposed diplexer is about 0.27λg × 0.11λg. The proposed diplexer show significant advantages in terms of size reduction, low loss, high selectivity, high Q-factor, easy bandpass frequency shifting, easy fabrication and, easy integration with other planar microwave circuits.

Transition Metal Dichalcogenide-Based Field-Effect Transistors for Analog/Mixed- Signal Applications

Transition metal dichalcogenides (TMDs), such as MoS2, MoSe2, MoTe2, WS2, WSe2, etc., have been considered as the most promising candidates for energy-efficient information processing at ultrascaled devices due to their decent energy gap of around 1–2 eV and single-atomic thickness. Even though there are many efforts to explore their performance for digital applications, their performance considerations for analog/mixed-signal applications are still unexplored. In this regard, we have assessed the analog/RF performance of TMD-based field-effect transistors (TMD-FETs) and investigated their benefits over graphene-FET and black phosphorous-FETs. The performance analysis is done by an in-house developed code, which involves the self-consistent solutions of 2-D Poisson’s equation and nonequilibrium Green’s function (NEGF) formalism. The results show that MoS2-FET can offer high intrinsic gain with the intrinsic cutoff frequency and maximum oscillation frequency in terahertz range. However, the significant degradation in high-frequency performance of MoS2-FET is observed in the presence of external resistances and parasitic capacitances. The cutoff frequency has found a few hundreds of gigahertz range in the presence of all parasitic conditions. It has also found that, among TMD-FETs, WSe2-FET could be a promising candidate for analog/RF integrated circuits with a higher drive current, intrinsic gain, cutoff frequency, and maximum oscillation frequency.

Impact of phonon scattering on digital characteristics and RF performance of graphene nanoribbon FETs

The switching capabilities and RF performance of graphene nanoribbon field-effect transistors (GNRFETs) in the presence of electron-phonon scattering are examined. Self-consistent atomistic simulations based on the nonequilibrium Green's function (NEGF) formalisms are used. We have studied the influence of incoherent transport on the most important digital parameters such as Ion, Ioff, Ion/Ioff, the subthreshold swing, and the drain-induced barrier lowering (DIBL) versus the channel length. Furthermore, the effects of phonon scattering on the gate capacitance and the transconductance are investigated in order to calculate the intrinsic cut-off frequency, switching delay, and the power-delay product of non-ballistic GNRFETs. We have shown that band-to-band tunneling regime is dominated by the phonon-assisted tunneling. Therefore, the off-current and the sub-threshold swing are deteriorated by this phenomenon. Furthermore, the scattering results in a decrease of intrinsic cut-off frequency and increase of the gate delay and energy consumed per switching event.